1. Field of the Invention
This invention relates to circuit breakers for high voltage electric power transmission and distribution systems, and more particularly, to circuit breakers having multiple series connected vacuum interrupters immersed in a tank of electrically insulating fluid with an isolation gap to prevent delayed restrikes in the vacuum interrupters. Such circuit breaker structures are particularly useful in uprating existing oil circuit breakers.
2. Background Information
Oil circuit breakers commonly used in electric power transmission and distribution systems have a pair of breaker units immersed side by side in a tank of high dielectric strength oil Typically, each breaker unit has multiple sets of contacts in series The contacts are normally held closed by electrically conductive laterally extending arms of a cross-bar carried by a lift rod held in a raised position between the breaker units by an external operator. This cross-bar also connects the two interrupter circuit breaker units in series when in the raised position. When the breaker is tripped, the lift rod is driven downward by the external operator. Initial downward travel of the cross-arm with the lift rod permits spring operated mechanisms of various design within the breaker units to open the contacts thereby striking arcs. Heat generated by these arcs causes a build-up of pressure which forces oil through a grid to deionize the arcs and interrupt the flow of current. The cross-bar maintains electrical contact with the interrupter assemblies until the current has been interrupted and then separates to form a pair of isolation gaps.
There is an interest in uprating these existing oil circuit breakers to obtain increased interrupting ratings and in addition or alternatively to extend the usable life of the breaker to increase reliability and to reduce maintenance. Conventional methods of uprating oil circuit breakers require major redesigns to the interrupters, tank and operating mechanism to survive the higher explosive forces developed when interrupting and closing in to higher current faults.
Vacuum interrupters are also widely used in electric power systems. However, vacuum interrupters have an intrinsic weakness to lightning impulses, and have a tendency, when switching capacitors, to have a delayed restrike after a successful interruption. As a result, the use of vacuum interrupters has been limited to lower voltage applications. A high voltage circuit breaker in which multiple vacuum interrupters are connected in series within a tank filled with SF.sub.6 is proposed in U.S. Pat. No. 3,839,612. However, this breaker requires too many vacuum interrupters to be economically feasible, and is not suitable for uprating present oil circuit breakers. Other arrangements for circuit breakers comprising multiple series connected vacuum interrupters are disclosed in U.S. Pat. Nos. 3,814,885 and 4,225,761. Again, these designs are not suitable for uprating current oil circuit breakers to forty and fifty kva ratings.
It is the primary object of the invention, therefore, to provide a means for modifying existing oil circuit breakers to increase their interrupting capability and in addition or alternatively to improve their reliability, and reduce the maintenance required to maintain them.
It is also an important object of the invention to provide such an improved circuit breaker through uprating of current oil circuit breakers.
It is a further object of the invention to provide such an improved circuit breaker which does not require redesign of and which ideally can utilize many of the components in existing oil circuit breakers.
It is an additional object of the invention to provide such an improved circuit breaker which utilizes vacuum interrupters.
It is another object of the invention to provide such an improved circuit breaker which eliminates the problem of delayed restrikes in vacuum interrupters.
It is yet another object of the invention to provide such an improved circuit breaker which comprises modular interrupter units which may be substituted for the oil breaker units in existing oil circuit breakers.
It is still another object of the invention to provide such an improved circuit breaker which applies an impact force to open the contacts of the vacuum interrupters utilizing the operator of the existing oil circuit breakers.